1. Field of the Invention
The present invention relates to a lightweight image- reading device comprising a lens array and plastic substrates contained in a case, the device being capable of reading images with a high reading accuracy and of being assembled by an automatic assembling process.
2. Description of the Related Art
The inventors of the present invention succeeded in developing plastic cylindrical lenses having a uniform refractive index distribution and having a uniform conjugate length, as disclosed in WO 09/05274. These plastic cylindrical lenses are formed of filaments produced by a true conjugate spinning process for continuously spinning a plurality of plastic compounds having different refractive indices. The uniformity of the performance of these cylindrical lenses having a uniform refractive index distribution is higher than that of glass cylindrical lenses, and thus the distributed refractive index cylindrical lens unit array comprising plastic cylindrical lenses has a superior uniformity of performance when compared to a distributed refractive index cylindrical lens unit array comprising glass cylindrical lenses.
The distributed refractive index cylindrical lens unit array comprising plastic cylindrical lenses is lightweight, which is advantageous for an application thereof to an image-reading device for copy machines and facsimile terminal equipment, which must be compact and have a lightweight construction.
FIG. 1 is a sectional view of a prior art image reading device constructed by placing a lens unit array 14 formed by adhering an array of a plurality of distributed refractive index cylindrical lens elements between two plates 12 and 13 held in a case 11. The case 11 is usually formed by die-casting or drawing light metals such as aluminum. In FIG. 1, 15 is a CCD sensor for converting image information read by a lens unit array 14 into corresponding electric signals; 16 is a light source; 17 is a cover glass; 18 and 18' are screws for securing the lens unit array 14 in case 11, and 19 and 19' are projections on which the lens unit array 14 is fixed.
The rapid progress in the reduction in weight and size of copying machines and facsimile terminal equipment requires the use of an image reading device having a lightweight construction and comprising a lens unit array having a fixed conjugate length, since the image reading device is to be incorporated into lightweight, compact copying machines and facsimile terminal equipment. To meet such requirements, an image reading device composed of a lens unit array formed by adhering an array of distributed refractive index cylindrical lens elements between plastic plates held in a case formed by die-casting aluminum has been developed. The MTF (modulation transfer factor) of this image reading device, measured by using a grid of four pairs of lines per millimeter, was as high as 50% or above, but this MTF dropped sharply to 30% or below when the MTF was measured after subjecting the image reading device to a heat cycle test as shown in FIG. 3, thereby proving that the performance of the image reading device is not stable.